After selection, divide by who's left
- The idea
- Predict
- See it
- Why
- Check
The idea
“A variant's new share is its survivors over the starting total, or just its survival rate.”
The starting total is printed right in the question, so it's the easy number to divide by. After selection, the population is only the ones still alive.
The short version
The mistake: A variant's new share is its survivors over the starting total, or just its survival rate.
What's actually true: After selection, a variant's share is its number of survivors divided by the total number of survivors.
- After selection, a variant's share is its survivors divided by all the survivors. The ones that died are gone, so they leave the total too.
- A survival rate compares a group with its own start. Three of 4 dark moths lived, a 75 percent survival rate. Their share is a different number.
- Alleles work the same way. Count the allele copies the survivors carry. In a diploid, divide by 2 times the number of survivors.
On the exam: After selection, count who's left. Divide the variant's survivors by all the survivors.